The present invention relates to a transport system. More particularly this invention concerns a transport method and apparatus used in a system that produces rigid sandwich panels of synthetic-resin material.
It is known to produce a rigid panel by injecting a mass of foamable synthetic-resin material between a pair of foils or skins. These skins are held apart at a predetermined fixed spacing while the synthetic-resin mass foams and completely fills the space between them. During such foaming it is essential that the skins be held perfectly flat so that the finished board or panel will also be perfectly flat.
In the continuous production of such boards the foils or skins are pulled off of supply rolls and then pulled between a so-called two-belt transport system. Such a system comprises a pair of endless belts made of hard synthetic-resin material or of steel which are moved in the production or transport direction at a predetermined speed and which have stretches that are exactly parallel to each other and spaced apart by the desired panel thickness. Since the pressure exerted by the foaming synthetic-resin material between the skins is considerable it is necessary to support the juxtaposed stretches of the transport belt by means such as rollers as described in the publication Kunststoffe (1968, volume 2, page 120). In such an arrangement an array of rollers extending perpendicular to the transport direction engage against the stretches on the faces thereof opposite the workpiece. These rollers are driven by the transport belt at a peripheral speed corresponding to the production rate so that the working stretches of the transport belts are firmly supported.
Such a system has the disadvantage that, since the contact between each of the rollers and the back of the corresponding stretch of the corresponding transport belt is effectively line contact, there are still unsupported regions of the belt stretches. It is therefore possible for the belt to bow out or deform outwardly in these unsupported regions. Such bowing-out can be minimized by placing the rollers as close as is physically possible. It is further possible to minimize the effects of such deformation by operating with the greatest possible transport speed. Thus between the time when the synthetic-resin mass is relatively plastic within the panel and the time when it is completely rigid, enough of the rollers will have been passed over to prevent the formation of excessive ripples or the like. Nevertheless it is noted that in some fast-setting resins the surface of the board will come out with a rippled surface.
In case the production line must be shut down temporarily in such a system the board that is at the moment pinched between the working stretches of the twin-belt transport system invariably has such a heavily rippled surfaces that it must be discarded. It is also noted that occasionally the extreme pressure exerted by the synthetic-resin mass will form ridges that are capable of damaging the machine when it is started up again.